I. Field of the Invention
The present invention is generally directed to a method and apparatus for fabricating a micro-electro-mechanical (MEM) device. More particularly, the present invention is generally directed to a method and apparatus for providing an encapsulated MEM device. Aspects of the invention are also particularly useful in providing a MEM apparatus comprising a two arm lever mechanism having increased rigidity. Such a lever mechanism may be used with a MEM switch, relay, sensor, actuator, accelerometer, and other like MEM device. Other aspects of the invention are also particularly useful in providing a MEM apparatus comprising an abrasion resistive contact that is preferably deposited along a contact area of the MEM device. However, certain aspects of the invention may be equally applicable in other scenarios as well.
II. Description of Related Technology
Micro-electro-mechanical devices (MEM devices) generally involve the integration of mechanical elements, actuators, sensors, and electronics on a common substrate. Ordinarily, such integration can occur through the use of micro-fabrication techniques. MEM devices can range in size from as small as a few microns to as large as a few millimeters. While the electronics that these MEM devices utilize are fabricated using Integrated Circuit (IC) process sequences (e.g., CMOS, Bipolar, or BiCMOS processes), micro-mechanical components can be fabricated using compatible “micro-machining” processes that selectively etch away portions of materials deposited on a substrate. Alternatively, the micro-machining process adds additional structural layers to form mechanical and electromechanical devices.
MEM devices bring together silicon-based microelectronics with micro-machining technology, thereby making possible the realization of a complete system-on-a-single substrate. MEM devices augment the computational ability of microelectronics with the perception and control capabilities of microsensors and/or microactuators. Examples of such electrical and mechanical combinations are gyroscopes, accelerometers, micro-motors, and sensors of micrometric size, all of which may need to be left free to move after some type of encapsulation and/or packaging. MEM devices may be used within digital to analog converters, air bag sensors, logic, memory, microcontrollers, and video controllers. Example applications of MEM devices are military electronics, commercial electronics, automotive electronics, and telecommunications.
MEM devices are essentially a technology used to create micro-miniature mechanical devices (such devices can be manufactured out of silicon or, alternatively, other materials). Ordinarily, these MEM devices are designed to respond to external stimuli. For example, where certain MEM devices are used for sensing applications, they can be fabricated so as to respond to the stimuli, and move (or actuate) mechanical structures. Known MEM technology is being applied to accelerometers in automobile airbags, pressure sensors, flow rate sensors, and other such like applications. Micro-mechanical micro arrays have also been developed for projection display applications. As will be discussed with respect to FIG. 1, MEM devices may sometimes be based on integrated circuit fabrication technologies such as those technologies similar to CMOS, with the added ability to incorporate moving and mechanical structures. Known MEM devices can typically range in size from one micron to several hundreds microns.
Certain known problems exist with existing MEM devices. MEM device are known to suffer from several types of problems. For example, one such problem involves the fabrication of MEM devices on top of a CMOS type device. An example of a known MEM device 10 is illustrated in FIG. 1. FIG. 1 illustrates a cantilever beam 12 designed over a CMOS device 16. As can be seen in FIG. 1, this MEM 10 includes a cantilevered beam 12 that generally follows the contour 14 of the underlying CMOS device 16. Therefore, the design and orientation of cantilever beam is not one that can be customized based on the specifics of the application. Rather, the cantilever beam shape will be generally dictated by the topology of the substrate layers residing underneath the cantilever beam.
In addition, with cantilevers residing over a CMOS topography, the shape of the cantilever will naturally be defined by the underlying topography of the substrate unless it is planarized before MEM fabrication. Consequently, such contoured beams will have a tendency to possess a non-uniform intrinsic stress distribution because of the device structure topography. In addition, such non-uniform cantilevers have different zero-load deflections.
One method that attempts to reduce these concerns with MEM devices designed over CMOS has been to design the substrate such that the MEM devices are located in a substrate location where no CMOS processing takes place. However, isolating these MEM devices to only a restricted substrate area can pose certain fabrication issues. One such issue relates to limiting the number of MEM devices per substrate. Isolating these MEM devices to a specific substrate area can also place certain restrictions on the applications for such a substrate.
There are other concerns arising from other known MEM devices. For example, with other known MEM devices, such devices often comprise a uniform or uni-planar cantilever beam. For example, one such known problem that arises in the fabrication of MEM devices from surface micromachining is that the cantilever does not have enough rigidity to return to the “off” position. This issue may be amplified where “stiction” occurs. Stiction usually arises when surface adhesion forces are higher than the mechanical restoring force of the micro-structure: the cantilever.
Stiction can also arise during the fabrication process. For example, when a MEM device is removed from an aqueous solution after wet etching of an underlying sacrificial layer, the liquid meniscus formed on hydrophilic surfaces can pull the microstructure towards the substrate. This pulling action results in what is known in the art as stiction.
In use, stiction may be caused by capillary forces, electrostatic attraction, and direct chemical bonding.
Another known approach to resolving the stiction issue relates to applying an anti-stiction coating to the MEM device. However, using anti-stiction coatings has other related issues. For example, these known anti-stiction coating approaches eventually degrade, particularly while the device is operating at high temperatures. In addition, certain anti-stiction coatings also have a limited service life.
MEM devices can find application as switches and relays in RF and microwave communication circuit such as transmit/receive switches, reconfigurable antennas, multiband switches, and signal routers. These types of switching devices may also find application in low frequency logic circuits. When a MEM device operates as a switch, the signal circuit is directly coupled with the activation circuit. In a MEM device operating as a relay, the two circuits are decoupled. For frequencies below about 2 GHz, the switches and relays are usually of a contact type. However, above 2 GHz, the switches and relays can be indirect switches since at these frequencies the impedance is rather small when coupling through a thin insulator layer. The impedance is given by Z=½πƒC with ƒ the frequency of the signal and C the capacitance of the swith contact.
Reliability issues often arise with such switches/relays. For example, contacts having increased reliability should be abrasion resistant. In addition, switches/relays having increased reliability should have contacts that do not deform from micro-arcs at high current densities, such as densities on the order of 1×106 A/cm2. These switches/relays should also operate at currents of up to 100 milli-Amps. For example, where a MEM device has a relay contact area on the order of 20×20 μm2 this corresponds to a current density of 2.5×104 A/cm2. However, since the surface of the switch/relay contact is not entirely smooth, local current densities at certain “hot” spots can be significantly higher. Consequently, there is a general need to provide a method and apparatus for reducing such current hot spots and for providing a contact system that increases the operating reliability of the switch/relay.
Another issue that is often faced by MEM device manufacturers that affects product yields is the potential contamination of MEM devices. For example, under ordinary operation, MEM devices are often placed in operating environments that have a certain amount of air-born contaminants, such as dust. Consequently, as a result of the micron-size of typical MEM devices (and therefore the micron-size movable components of such devices), dust, various processing fluids, etchants, or other fluid and/or air-born contaminants pose a threat to the efficient operation of the MEM devices.
Another problem that may be associated with failure rates relates to MEM lever mechanism beam rigidity. Consequently, there may also be a general need for a MEM device having increased rigidity. Based in part on these forgoing issues, there is, therefore, a general need to be able to increase the production yield of MEM devices deposited on a substrate. There is also a general need to reduce the amount of contamination that a MEM device may experience, either during device design, device fabrication, device operation, device packaging, or otherwise. There is also a general need to decrease the failure rate of a MEM device caused in part by contamination, stiction, or other operating concerns. These and other general needs should also be met while fabricating a MEM device having uniform mechanical lever stress for switching purposes.